1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation of the type having a luminescent storage screen for the latent storage of x-ray images, and a read-out system for the screen.
2. Description of the Prior Art
An x-ray diagnostics apparatus is disclosed in European Application 0 387 369, corresponding to the U.S. Pat. No. 5,049,749, wherein a luminescent storage screen in an x-ray diagnostics apparatus is irradiated with x-rays in an exposure station so as to produce a latent x-ray image in the form of a pattern of electron holes stored in potential traps in the phosphor layer of the screen. Subsequently, the exposed luminescent storage screen is conveyed to a read-out station, wherein the entire surface of the storage screen is scanned pixel-by-pixel with a separate radiation source, for example a laser, so that the holes stored in the traps are excited and can fall back to a recombination state, with the energy difference being emitted in the form of light quanta having respectively different wavelengths compared to the wavelength of the radiation source. The light emitted in this manner is acquired by a detector, so that the stored x-ray image is read out from the screen. This image is then supplied in a known manner to an image reproduction system, which either displays the image on a monitor, or produces a hard copy of the image, or both.
For planar scanning of the luminescent storage screen, the laser beam, for example, is deflected in the horizontal direction by a holographic deflection device, and is also shifted in the vertical direction, so that all of the picture elements on the luminescent storage screen are excited in succession. The light emitted by the luminescent storage screen is acquired by an optical collector, and is imaged on the light-sensitive input surface of a detector, or a plurality of detectors. The output signal from the detector is supplied, for example, to a conventional video chain for reproduction of the x-ray image on a monitor.
As described in German Patent 3 347 207, corresponding to U.S. Pat. No. 4,587,036, europium-activated barium fluoro-bromine chloride compounds can be excited by visible light, and can be used as stimulable phosphors in storage screens of this type. For exciting this stimulable phosphor, a standard He-Ne laser can be used, which generates a focused beam with a wavelength of 633 nm.
Such known x-ray diagnostics systems, however, have the disadvantage that fluctuations in the output signal of the detector may be generated which are not due to brightness differences in the x-ray image, but are instead caused by fluctuations in the intensity of the laser beam. Moreover, non-uniformities in the structure of the detector may cause the generation of a non-linear output signal, which varies dependent on the position of the scan position of the luminescent storage screen, i.e., on the deflection of the scan beam.